The IEEE 802.11 MAC standard defines the way Wireless local area networks (WLANs) must work at the physical and medium access control (MAC) level. Typically, the 802.11 MAC (Medium Access Control) operating mode implements the well-known Distributed Coordination Function (DCF) which relies on a contention-based mechanism based on the so-called “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA) technique.
The 802.11 medium access protocol standard or operating mode is mainly directed to the management of communication nodes waiting for the wireless medium to become idle so as to try to access to the wireless medium.
The network operating mode defined by the IEEE 802.11ac standard provides very high throughput (VHT) by, among other means, moving from the 2.4 GHz band which is deemed to be highly susceptible to interference to the 5 GHz band, thereby allowing for wider frequency contiguous channels of 80 MHz to be used, two of which may optionally be combined to get a 160 MHz channel as operating band of the wireless network.
The 802.11ac standard also tweaks control frames such as the Request-To-Send (RTS) and Clear-To-Send (CTS) frames to allow for composite channels of varying and predefined bandwidths of 20, 40 or 80 MHz, the composite channels being made of one or more channels that are contiguous within the operating band. The 160 MHz composite channel is possible by the combination of two 80 MHz composite channels within the 160 MHz operating band. The control frames specify the channel width (bandwidth) for the targeted composite channel.
A composite channel therefore consists of a primary channel on which a given node performs EDCA backoff procedure to access the medium, and of at least one secondary channel, of for example 20 MHz each. The primary channel is used by the communication nodes to sense whether or not the channel is idle, and the primary channel can be extended using the secondary channel or channels to form a composite channel.
Sensing of channel idleness is made using CCA (clear channel assessment), and more particularly CCA-ED, standing for CCA-Energy Detect. CCA-ED is the ability of any node to detect non-802.11 energy in a channel and back off data transmission. An ED threshold based in which the energy detected on the channel is compared is for instance defined to be 20 dB above the minimum sensitivity of the PHY layer of the node. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy becomes below the threshold anew.
Given a tree breakdown of the operating band into elementary 20 MHz channels, some secondary channels are named tertiary or quaternary channels.
In 802.11ac, all the transmissions, and thus the possible composite channels, include the primary channel. This is because the nodes perform full Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) and Network Allocation Vector (NAV) tracking on the primary channel only. The other channels are assigned as secondary channels, on which the nodes have only capability of CCA (clear channel assessment), i.e. detection of an idle or busy state/status of said secondary channel.
An issue with the use of composite channels as defined in the 802.11n or 802.11ac (or 802.11ax) is that the 802.11n and 802.11ac-compliant nodes (i.e. HT nodes standing for High Throughput nodes) and the other legacy nodes (i.e. non-HT nodes compliant only with for instance 802.11a/b/g) have to co-exist within the same wireless network and thus have to share the 20 MHz channels.
To cope with this issue, the 802.11n and 802.11ac standards provide the possibility to duplicate control frames (e.g. RTS/CTS or CTS-to-Self or ACK frames to acknowledge correct or erroneous reception of the sent data) in an 802.11a legacy format (called as “non-HT”) to establish a protection of the requested TXOP over the whole composite channel.
This is for any legacy 802.11a node that uses any of the 20 MHz channel involved in the composite channel to be aware of on-going communications on the 20 MHz channel. As a result, the legacy node is prevented from initiating a new transmission until the end of the current composite channel TXOP granted to an 802.11n/ac node.
As originally proposed by 802.11n, a duplication of conventional 802.11a or “non-HT” transmission is provided to allow the two identical 20 MHz non-HT control frames to be sent simultaneously on both the primary and secondary channels forming the used composite channel.
This approach has been widened for 802.11ac to allow duplication over the channels forming an 80 MHz or 160 MHz composite channel. In the remainder of the present document, the “duplicated non-HT frame” or “duplicated non-HT control frame” or “duplicated control frame” means that the node device duplicates the conventional or “non-HT” transmission of a given control frame over secondary 20 MHz channel(s) of the (40 MHz 80 MHz or 160 MHz) operating band.
In practice, to request a composite channel (equal to or greater than 40 MHz) for a new TXOP, an 802.11n/ac node does an EDCA backoff procedure in the primary 20 MHz channel. In parallel, it performs a channel sensing mechanism, such as a Clear-Channel-Assessment (CCA) signal detection, on the secondary channels to detect the secondary channel or channels that are idle (channel state/status is “idle”) during a PIFS interval before the start of the new TXOP (i.e. before the backoff counter expires).
More recently, Institute of Electrical and Electronics Engineers (IEEE) officially approved the 802.11ax task group, as the successor of 802.11ac. The primary goal of the 802.11ax task group consists in seeking for an improvement in data speed to wireless communicating devices used in dense deployment scenarios.
Recent developments in the 802.11ax standard sought to optimize usage of the composite channel by multiple nodes in a wireless network having an access point (AP). Indeed, typical contents have important amount of data, for instance related to high-definition audio-visual real-time and interactive content. Furthermore, it is well-known that the performance of the CSMA/CA protocol used in the IEEE 802.11 standard deteriorates rapidly as the number of nodes and the amount of traffic increase, i.e. in dense WLAN scenarios.
In this context, multi-user transmission has been considered to allow multiple simultaneous transmissions to/from different users in both downlink and uplink directions. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple nodes to simultaneously transmit.
To actually perform such multi-user transmission, it has been proposed to split a granted 20 MHz channel into sub-channels (elementary sub-channels), also referred to as resource units (RUs), that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.
OFDMA is a multi-user variation of OFDM which has emerged as a new key technology to improve efficiency in advanced infrastructure-based wireless networks. It combines OFDM on the physical layer with Frequency Division Multiple Access (FDMA) on the MAC layer, allowing different subcarriers to be assigned to different nodes in order to increase concurrency. Adjacent sub-carriers often experience similar channel conditions and are thus grouped to sub-channels: an OFDMA sub-channel or RU is thus a set of sub-carriers.
As currently envisaged, the granularity of such OFDMA sub-channels is finer than the original 20 MHz channel band. Typically, a 2 MHz or 5 MHz sub-channel may be contemplated as a minimal width, therefore defining for instance 9 sub-channels or resource units within a single 20 MHz channel.
To support multi-user uplink, i.e. uplink transmission to the 802.11ax access point (AP) during the granted TxOP, the 802.11ax AP has to provide signalling information for the legacy nodes (non-802.11ax nodes) to set their NAV and for the 802.11ax nodes to determine the allocation of the resource units RUs.
It has been proposed for the AP to send a trigger frame (TF) to the 802.11ax nodes to trigger uplink communications.
The document IEEE 802.11-15/0365 proposes that a ‘Trigger’ frame (TF) is sent by the AP to solicit the transmission of uplink (UL) Multi-User (OFDMA) PPDU from multiple nodes. In response, the nodes transmit UL MU (OFDMA) PPDU as immediate response to the Trigger frame. All transmitters can send data at the same time, but using disjoint sets of RUs, resulting in transmissions with less interference.
The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is added. The TF frame is sent over the primary 20 MHz channel and duplicated (replicated) on each other 20 MHz channels forming the targeted composite channel. As described above for the duplication of control frames, it is expected that every nearby legacy node (non-HT or 802.11ac nodes) receiving the TF on its primary channel, then sets its NAV to the value specified in the TF frame in order. This prevents these legacy nodes from accessing the channels of the targeted composite channel during the TXOP.
A resource unit RU can be reserved for a specific node, in which case the AP indicates, in the TF, the node to which the RU is reserved. Such RU is called Scheduled RU. The nodes do not need to perform contention on accessing scheduled RUs.
In order to better improve the efficiency of the system in regards to unmanaged traffic to the AP (for example, uplink management frames from associated nodes, unassociated nodes intending to reach an AP, or simply unmanaged data traffic), the document IEEE 802.11-15/0604 proposes a new trigger frame (TF-R) above the previous UL MU procedure, allowing random access onto the OFDMA TXOP. In other words, the resource unit RU can be randomly accessed by more than one node. Such RU is called Random RU and is indicated as such in the TF. Random RUs may serve as a basis for contention between nodes willing to access the communication medium for sending data.
The random resource selection procedure is not yet defined. All that is known is that the trigger frame may define only Scheduled RUs, or only Random RUs within the targeted composite channel.
There is no guarantee that the Scheduled or Random RUs will be used by the nodes.
It is particularly the case for the Random RUs because any rule used by the nodes to select a Random RU may result in having RUs not allocated at all to any node. Also, the AP does not know whether or not some nodes need bandwidth. In addition, some RUs provided by the AP may not be accessible for some nodes because of hidden legacy nodes.
It is also the case for the Scheduled RUs (which are reserved by the AP because some nodes have explicitly requested bandwidth) if the specified nodes do not send data.
It results that the channel bandwidth is not optimally used.
Furthermore, the more unused RUs within a 20 MHz channel, the lower the average energy over this 20 MHz channel.
However, since the legacy nodes not registered to the AP use this average energy over their primary 20 MHz to sense whether it is idle or busy, the presence of unused RUs increases the risk that legacy nodes sense the corresponding 20 MHz channel as idle. The legacy nodes may then transmit data on this 20 MHz channel, thus colliding with the data traffic conveyed over the used RUs.